A known optical scanning device comprises a light source, a deflector, e.g., a polygon mirror, configured to deflect light emitted from the light source, and a light detector configured to detect the light deflected by the deflector. The light detector is disposed in an upstream position in a scanning direction of the deflected light. In another known scanning device for use in a color printer, four light sources, two polygon mirrors, and four light detectors are disposed in a box-shaped housing.